Pluto is at opposition this month, meaning that it’s now at its brightest.
That’s not good enough. It’s too dim to appear in even the finest binoculars. Even a large amateur telescope merely shows it as a faint speck, and you’d need a super-accurate star chart to distinguish it from the zillions of other faint dots in Sagittarius, in the direction of the galaxy’s center. Pluto shines feebly at 14th magnitude. It’s 600 times fainter than the dimmest naked-eye stars.
It’s dim because it’s not just far away; it’s tiny – roughly half the size of our Moon. But it’s not an “it.” It’s a “they.” There are two of them, dual balls with a mere two-to-one size difference: Pluto and Charon (pronounced like the feminine name Karen). The pair orbit around an empty piece of space between them, once a week. Both worlds rotate too, so that just one hemisphere of each always faces the other, eyeball-to-eyeball. Strange stuff.
No spacecraft has ever visited its frozen surface – but that’s about to change. Exactly a year from now, after spending eight lonely years en route, the fastest-ever spacecraft will arrive there. Next summer, when the New Horizons mission zooms past Pluto (it cannot stop and orbit), the tiny blurry world will suddenly blossom into glorious new detail. So this is our last year of it being truly mysterious.
As we all know, in 2006 the International Astronomical Union, the world body that decides such things, demoted Pluto to a dwarf planet. It was probably the right call. The first problem is that Pluto is far smaller than any other planet, with only four percent of the mass of even tiny Mercury. Our Earth, which weighs 81 times more than the Moon, weighs 2,000 times more than Pluto. The second issue is that it has a very unplanetlike orbit, from every angle.
But astronomers could have lived with those things. The clincher was finding more Plutos out there. Eris is even bigger than Pluto, while Makemake, Quaoar, Sedna and a few others are almost as large. If Pluto’s a planet, then these others must be, too. Eventually we’d have dozens of tiny icy planets that are all very different from the original eight. It became clear that there’s a Kuiper Belt out there with thousands of small, icy unplanetlike bodies, and Pluto’s one of them. It’s really a Kuiper Belt Object: a KBO.
But astronomers ultimately decided not to label Pluto a KBO. Instead, they created a new category, and called it a dwarf planet. They threw a few other celestial bodies into that bin, too – including the largest asteroid Ceres, which orbits right here in the inner solar system. That’s because Ceres and Pluto are both spherical. So you’re free to call Ceres an asteroid, a dwarf planet or a minor planet. No wonder people find astronomy confusing. (By the way, Ceres is now meeting the brightest asteroid, Vesta, in the constellation Virgo. This is quite a rare encounter.)
Pluto remains a popular world, and the public is unhappy with the decision to demote it from the ranks of major planets. I think that’s largely because everyone likes its name. I’ll bet folks wouldn’t be as sorry to see Uranus bite the dust.
It’s no coincidence that the cartoon dog has the same name as the planet, or ex-planet. Actually, Walt Disney’s only non-speaking character was originally named Rover in an earlier cartoon. In 1931, a year after Pluto’s discovery, the Disney folks decided to exploit the global publicity of that newly found world, and changed the character’s name.
Maybe they should now keep pace with the latest changes and make the cartoon character a “dwarf dog.”
Want to know more? To read Bob Berman’s previous “Night Sky” columns, visit our Almanac Weekly website at HudsonValleyAlmanacWeekly.com.